Polymeric blends comprising at least one elastomeric polymer and at least one nonelastomeric polymer are known in the prior art. For example, U.S. Pat. No. 4,804,577 to Hazelton et al., which is incorporated herein by reference in its entirety, discloses a nonwoven web prepared by meltblowing fibers from a polymeric blend comprised of a highly viscous elastomer with a less viscous thermoplastic resin. The webs may be used in making various clothing articles such as shoes and protective clothing as well as tarpaulins and tents. The elastomers disclosed for use in this process include polyolefin rubbers such as polyisobutylene, other olefin-rubbers, and elastomers prepared by polymerizing diolefins with various alkenyl aromatic hydrocarbons such as butadiene and styrene elastomers. The elastomeric polyolefin is present in the blend from about 5 to about 75% by weight and the thermoplastic polymeric resin is present in the blend from about 95 to about 25% by weight.
Hazelton et al. discloses that blends comprising more than about 10% by weight of the elastomeric polyolefin would be subjected to degradation prior to melt blowing. Such blends would typically exhibit viscosity above 500 poise at melt blowing conditions. Thermal and/or oxidative degradation would be performed to facilitate incorporation and dispersion of the highly viscous elastomer into the less viscous thermoplastic resin.
U.S. Pat. No. 5,597,194 to Daugherty et al. discloses a plastic net produced from a blend of plastics having different densities and peak melting points. The polymeric blend includes a polyethylene or a metallocene-catalyzed polyethylene component and a polyethylene plastomer component. The polyethylene plastomer component is formed using metallocene catalyst technology and are homopolymers of ethylene, or copolymers of ethylene with higher alphaolefins. The plastomers utilized are commercially available from Dow Plastics under various designations and trademarks such as ENGAGE and from Exxon Chemical Americas, Polymer Group under the trademark EXACT. The blend can be extruded to produce a high friction, non-slip, flexible and heat resistant plastic net that can be used as a truck bed liner.
U.S. Pat. No. 5,635,262 to Best et al., discloses articles made from a high molecular weight, high density polyethylene component and an elastomer component such as a styrenic copolymer, a rubber, or a metallocene-catalyzed polyethylene. The elastomer is present in the fabricated article from 1 to 15 weight percent. Articles include films, bags made from films, and extrusion blow-molded articles.
U.S. Pat. No. 5,110,685 to Cross et al. discloses a polymer coating consisting of a blend of high density polyethylene and ethylene/propylene rubber elastomers. U.S. Pat. No. 4,833,194 to Kuan et al. also discloses a blend of crystalline polymers such as polyethylene and elastomers such as ethylene-propylene copolymer or ethylene-propylene diene terpolymer.
Various methods and means of making and utilizing materials with elastic properties are known in the art. For example, U.S. Pat. No. 4,720,415 to Vander Wielen et al., which is incorporated herein by reference in its entirety, discloses a method of producing a composite elastic material. The method comprises stretching an elastic web, such as a nonwoven web of elastic fibers, and bonding the elongated web to one or more webs of gatherable material under conditions which soften at least a portion of the elastic web to form the bonded composite web of elastic material. The gatherable material, in one embodiment, may be a spunbonded polyester fiber material. The composite material is relaxed immediately after the bonding to prevent the elastic web from losing its ability to contract from the stretched dimensions that it assumes during the bonding step.
In one embodiment, Vander Wielen et al. utilizes polystyrene/poly(ethylene-butylene)/polystyrene block polymers that are available under the trademark KRATON from Shell Chemical Company. These KRATON rubber-type materials have been used to provide the elastic component for various articles including stretchable protective covers and wraps, outerwear, undergarments, menstrual and incontinence control articles and garments such as disposable diapers. KRATON materials are particularly useful in applications requiring a good deal of elastic memory such as baby diapers and incontinence control garments. The KRATON materials exhibit the ability to retract to near original forms after being stretched.
As indicated above, elasticity for various articles has also been provided by the metallocene-catalyzed polyethylenes. Dow Plastics, a division of Dow Chemical Company, has introduced several families of elastic polyolefins created by single-site, or constrained geometry, catalysts. For example, U.S. Pat. Nos. 5,472,775 to Obijeski et al., and 5,278,272 and 5,272,236 to Lai et al., which are incorporated herein in their entireties by reference thereto, describe various metallocene-catalyzed polyolefins produced by Dow Chemical. The materials have both plastic and rubbery characteristics.
Other companies, such as Exxon Chemicals, have also developed various processes for forming elastic polyolefins using metallocene-catalyzed synthesis. For example, U.S. Pat. No. 5,324,800 to Welborn, Jr. et al. describes several processes for forming such materials using metallocene catalysts, and is incorporated herein in its entirety by reference thereto.
When KRATON materials are used in various extrusion and meltblowing processes, flow modifiers must often be employed to relieve the intense die tip pressures that are present if only KRATON base rubber is used. For example, it is known to utilize a high melt flow polyethylene in conjunction with the KRATON base rubber in order to achieve a flowing polymer that is capable of being meltblown with conventional technologies.
In addition, problems are encountered in the prior art that employs blends of low density polyethylene with high molecular weight high density polyethylene. The differences in melt viscosity between the two polymers are such that non-homogeneity results in the melt and in the resulting film, thus leading to areas of good performance and areas of poor performance in the resulting nonwoven webs. Attempts to homogenize such mixtures to improve the dispersion and the overall physical properties have generally been unsuccessful.
The use of 100% metallocene-catalyzed polyethylene to form acceptable nonwoven webs through conventional meltblowing processes has also been tried. However, practical limitations have sometimes prevented acceptable commercial utilization of webs formed from such polymers. Due to the lower melting point of metallocene-catalyzed polyethylene (around 140° F. (60° C.)) and possibly due to the lower crystalline content, quenching is more difficult. As is known, typical meltblowing processes rely on ambient air being pulled toward the meltblowing jet to quench, and thus solidify, the meltblown fibers. Because the melting point of the metallocene-catalyzed polymers is lower, more quenching time or larger volumes of secondary air or quench water are required, thus making the entire meltblowing process more difficult and costly. Failure to properly quench or solidify the fibers will create the possibility that the webs will not release well from the wire upon which they are formed.
In addition, the use of 100% metallocene-catalyzed polymers in the meltblowing process often results in poor web formation. Due in part to the high viscosity and quenching problems discussed above, nonwoven webs formed from only elastomeric olefins, such as metallocene-catalyzed polyethylene, may be open or splotchy. In addition, the higher the viscosity of the polymer, the more difficult it is to produce small diameter microfibers which are necessary to achieve good web formation and coverage.
Another viscosity-related problem is encountered when using the more viscous metallocene-catalyzed resins. The more viscous a polymer is, the higher the pressure at the meltblowing die tip. This high die tip pressure limits the rate at which the fibers can be successfully manufactured.
Although use of the KRATON elastomers described above avoid some of the problems encountered when using 100% metallocene-catalyzed polymers, such rubber materials are extremely expensive. Typically, KRATON rubber materials will be as much as three times more expensive than comparable metallocene-catalyzed polyolefins. Although the use of KRATON materials results in webs having excellent elastic properties as well as articles having suitable “quilted” textures, the expense of such KRATON material generally prevents their use in cheaper, low end products such as wipers. Except for applications involving high end products such as diapers, the use of the KRATON materials is often inhibited because of the raw material cost involved.
There is, therefore, a need for a polyethylene composition that exhibits improved tensile strength and decreased viscosity, such that fabrication remains commercially practicable from an economic standpoint and at low elongations performed similarly. Because the metallocene-catalyzed polyolefins are much less expensive than the KRATON materials, it would be desirable to utilize the metallocene-catalyzed polyolefins over such materials.
While both polymeric blends of polyethylene and typical rubber-type elastomers and polymeric blends of various polymers and metallocene-catalyzed polyethylene are known in the art, the advantages arising from the unique combination of the present invention in the particular application as the elastic sheet(s) in various nonwoven laminates such as the stretch-bonded and neck-bonded laminates disclosed herein, have not heretofore been recognized.